Defense Secretary Harold Brown disclosed last night that U.S. intelligenc has discovered four new intercontinental ballistic missile systems under development in the Soviet Union.

Pentagon officials confirmed that he was referring to a completely new generation of ICBMs, not the present-day strategic nuclear missiles going into Soviet underground silos.

The same officials said last night that they could reveal no further details about the new Russian missile such as the size of their warheads how many were under development.

Brown's reference to the new missiles was the first official mention of them by the Carter administration, which is trying, with little apparent success, to negotiate a new arms control agreement with the Soviet Union.

The administration's disclosure about the existence of a fifth generation of Soviet ICBMs - beyond the present-day SS-16, SS-18 and SS-19 missiles - is expected to harden domestic opposition to President Carter's pending proposals to the Soviet Union for slowing the arms race.

"All of us must recognize," Brown told the 34th annual dinner of the National Security Industrial Association, comprised of representatives of defense firms, "that the Soviets have under way a number of large, impressive and costly strategic programs to strengthen their offensive capabilities, their active defense and their passive defense system.

"To give you just one indication of the effort and of the momentum behind it, the Soviets are now deploying a fourth generation of ICBMs at a rate of between 100 and 150 a year. These missiles are almost uniformly first class in terms of their accuracy and payload."

Then he made the disclosure in this offhand manner: "At the same time that the Soviets have four new ICBMs under development, they are continuing work on the SS-16, their mobile ICBM, and they are modifying four other missiles.

"Exactly why the Soviets are pushing so hard improve their strategicnuclear capabilities is uncertain."

Thomas B. Ross, assistant secretary of defense for public affairs, confirmed when queried last night that Brown was referring to the development of four hitherto undisclosed models of Soviet ICBMs.

"What is certain," Brown said in what was billed as a major policy speech, "is that we cannot ignore their efforts or assume that the Soviets are motivated by considerations of defense or even altruism."

To deter the Soviet Union from using its nuclear might," Brown pledged that the American strategic arsenal would continue to be powerful enough "to inflict unacceptable damage on the U.S.S.R. after an all-out Soviet first strike."

In discussing what the United States needs for less than all-out strategic nuclear war, Brown embraced the 1 1/2 war policy of the Nixon administration, declaring:

"We must continue to maintain a defense posture that permits us to respond effectively and simultaneously to a relatively minor, as well as to a major, military contigency."

Beyond Europe, Brown cited the Middle East, Persian Gulf and Korea as areas "where the United States and its allies have vital interests."

For those areas outside Europe, Brown said the United States needs "a limited number of relatively light land combat forces" not committed to NATO, "such as the three Marine divisions and some light Army divisions."

His speech may foreshadow a change in the Marines' role as a NATO reserve force. He said the Army's 2d Division will remain targeted on the Western Pacific after it leaves South Korea for a base in the United States.

While stressing the Soviet threat and other potential hot spots do not dictate crash program to buttress American defenses, Brown said that "in order to ensure deterrence, we plan to raise the level of U.S. defense spending by approximately 3 per cent a year in real terms."